Rochelle salt resonator



' Jul 13, 1948.

B. MATTHIAS ROCHELLE SALT RESONATOR Filed llay 31 1944 Patented July 13,1948 UNITED STATES PATENT OFFICE ROCHELLE SALT RESONATOR Bernd Matthias,Zurich, Switzerland, assignor to Patelhold Patentverwertungs- & Elektro-Holding A.-G., Glarus, Switzerland Application May 31, 1944, Serial No.538,142 In Switzerland April 12, 1943 3 Claims. 1

' tric crystals.

It is known that by means of the piezo-electric effect crystals can beexcited at their resonance frequency and used to stabilize highfrequency electrical oscillations in oscillation circuits. Up to thepresent quartz crystals have preferably been used for this purpose,although it is also known to use crystals of Rochelle salt in theirplace. It is also known to applya direct voltage to such a Rochellecrystal in order to change its resonance frequency. The practicalapplication of Rochelle crystals is, however, greatly handicapped by thelarge damping. This is primarily a drawback as regards generatorcircuits because it is impossible to obtain a sharp and stablefrequency. Filters with such crystals are not very selective.

A Rochelle electric crystal which is caused to oscillate can be regardedas the equivalent of an electrical oscillation circuit, which accordingto Fig. 1 consists of two capacitances C1 and C2, an inductance L and anohmic resistance R. This oscillation circuit possesses the disadvantagethat the resonance frequency is not sharply defined and the dielectriclosses are high. It is already known that theresonance frequency can bevaried by applying a direct voltage to both faces of the crystals. Thisdependence of the resonance frequency on the applied direct voltageexists over the range within which the polarisation of the crystal showsa hysteresis character as a function of the voltage applied to bothfaces; that is to say the dependence exists up to field strengths ofabout 900 v./cm. If the field strength is increased still furtheranother phenomenon occurs, namely for this range the electrical lossesare much smaller than in the range where the polarisation has ahysteresis character. By applying a. direct voltage the crystal becomesdeformed. Therefore conversely by means of deformation it is possible tocause the hysteresis character of the polarisation to disappearpractically. With these electrical or mechanical influences the piezomodulus remains at a value of about the same order of magnitude.

The present invention thus consists of an oscillation circuit for highfrequency electrical oscillations whose frequency is controlled by atleast one Rochelle electric crystal, the invention being characterisedby the feature that means are provided whereby the operating point is10- cated in a range within which the polarisation of the Rochelleelectric crystal is practically a deflnite function of the voltageapplied to both faces of the crystal.

Objects of the invention are to improve the selectivity or sharpness oftuning of oscillation circuits which include Rochelle salt crystals, the

increased selectivity being obtained through new methods of and circuitarrangements for reducing the dielectric losses in the crystals. Objectsare to provide oscillation circuits of the type including Rochelle saltcrystals which are preloaded or polarized electrically and/ormechanically substantially to eliminate the hysteresis character of theknown electrical polarization corresponding to biasing electrical fieldsof the order of up to about 900 volts per centimeter. Other objects areto provide methods of and apparatus for adjusting the frequency of anoscillation circuit which is frequency stabilized by a Rochelle saltcrystal pre-loaded to such a degree that the resonant frequency of thecircuit is substantially independent of small changes in polarizingvoltages applied to the crystal to pre-load the same.

These and other objects and the advantages of the invention will beapparent from the following specification when taken with theaccompanying drawing in which:

Fig. 1 is the equivalent circuit diagram of a. Rochelle salt crystal;and

Figs. 2 and 3 are circuit diagrams of two oscillation circuits embodyingthe invention.

Two constructional examples of the invention are illustrateddiagrammatically in Figs. 2 and 3 of the accompanying drawing, Fig. 2showing a crystal-controlled transmitter and Fig. 3 an electric filterwith two Rochelle crystals. In the arrangement shown in Fig. 2 theRochelle electric crystal 1 is separated from the transmitter by thecapacitances 2. In order to locate the operating point outside theaforementioned hysteresis region, so that the dielectric losses in thecrystal are small, and so that a sharp resonance curve is obtained, asuitable direct voltage is applied to both faces. This is supplied toterminals l by a voltage source 5. The direct voltage source isseparated from the crystal by the choke coils 3. These prevent highfrequency power from escaping over the direct voltage source. The feedback from the anode to the grid circuit occurs over the grid-anodecapacitance.

The dielectric losses not only depend on the initial voltage but also onthe temperature. They .attain a maximum value at the Curie points andhave a minimum value between these points.

It is therefore expedient to choose the operating point so that it liesin the region of this minimum value.

Fig. 3 shows how for example Rochelle electric crystals I are used in afilter arrangement of known type in which each crystal is shunted by avariable condenser 6 and the opposite faces of the crystals arecross-connected by variable condensers I. By means of the direct voltagesources the operating point on the Rochelle electric crystals is againlocated in a range where the losses practically disappear. The directvoltages I are separated from the crystals I by the high frequencyblocking elements 3. If necessary the Rochelle electric crystals canalso be separated from the rest of the high frequency arrangement.

In certain cases it may be an advantage to make the resonance frequencyof the crystals variable, for instance so as to make the passing rangeof a filter variable. For this purpose the direct voltage should bevaried in the hysteresis region, as already mentioned. It has, however,been determined that the resonance frequency also depends on themagnitude of the alternating voltage at the crystal surfaces, so thatthis voltage can also be employed for varying the frequency.

The dielectric losses are high when regulating with direct voltage.Furthermore the necessary alternating voltage alterations are muchsmaller than the corresponding direct voltage alterations. For thisreason it is often an advantage to control the frequency with thealternating voltage. The alteration of the frequency as a function ofthe alternating voltage amplitude is a maximum when the temperature isso selected that the operating point lies in the region of one of theCurie points, it being particularly advantageous when it is in theregion of the upper Curie point. Furthermore the temperature of thecrystal can also be employed for slow changes in frequency, because thefrequency follows a temperaturedependent course, whereby there is aminimum value at each Curie point and a maximum value between thesepoints.

The direct voltage source for the elimination of the hysteresis lossescan be obtained in several ways. Either a special direct voltage sourcecan be provided, or the direct voltage is produced by extracting highfrequency energy from the oscillation circuit and if necessarytransforming it to a higher voltage and then rectifying it. In order tomaintain the direct voltage exactly constant it is advisable tostabilize it.

If the frequency is to be kept as constant as possible, it is not onlynecessary to maintain the electrical values but also the temperatureconstant. Furthermore it is also an advantage to locate the operatingpoint in a range where the resonance frequency changes as little aspossible as a function of the temperature. For this purpose the regionof maximum frequency between both Curie points is especially favourable.It is, however, also possible to operate with the flattened parts at theCurie points. Furthermore in accordance with the present invention ithas been recognized that by applying a field strength exceeding 1400v./cm it is possible to achieve a considerable flattening of theresonance frequency curve as a function of the temperature, and thecurve becomes still flatter if the alternating voltage is less than 0.1volt.

The crystal becomes deformed when the direct voltage is applied. Bymeans of this deformation it is therefore possible to make thehysteresis losses disappear. A favorable constructional form of theinvention is therefore obtained when by deforming the Rochelle crystal,for instance by pressure, the operating point is located in a rangewhere the polarisation of the crystal is practically a definite functionof the voltage applied to both faces.

claim:

1. An electrical high frequency oscillation circult comprising at leastone Rochelle salt crystal for controlling the frequency of the circuit,means for impressing an alternating current voltage upon the crystal,and a direct current source for establishing in the crystal anelectrical field strength of the order of 1400 volts per centimeter.

2. In a filter, in combination, a circuit comprising a pair ofconductors, a Rochelle electrical crystal in series with each conductor,and means polarizing said crystals to locate the operating pointsthereof outside the range within which the crystals'exhibit a hysteresischaracteristic as a function of the applied polarization, saidpolarizing means comprising a direct voltage source individual to eachcrystal and connected to impress a voltage across the opposite facesthereof.

3. The invention, as set forth in claim 2, wherein a high frequencyblocking element is interposed between each voltage source and thecrystal associated therewith.

.BERND MA'I'I'HIAS.

REFERENCES CITED The following references file of this patent:

UNITED STATES PATENTS are of record in the Number Name Date 1,994,220Osnos Mar. 12, 1935 1,996,504 Darlington Apr. 2, 1935 2,306,555 MuellerDec. 29, 1942 OTHER REFERENCES

